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LOGICAL FALLACIES. Common Mistakes in Weak Arguments. DEVELOPED BY JENIFFER VISCARRA. Definition. Logical fallacies are flaws in reasoning that lead to faulty, illogical statements. They are unreasonable argumentative tactics named for what has gone wrong during the reasoning process. .

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logical fallacies


Common Mistakes in Weak Arguments



Logical fallacies are flaws in reasoning that lead to faulty, illogical statements. They are unreasonable argumentative tactics named for what has gone wrong during the reasoning process.

Most logical fallacies masquerade as reasonable statements, but they are in fact attempts to manipulate readers by reaching their emotions instead of their intellects.
i deduce it was miss scarlett
I deduce it was Miss Scarlett…
  • all inductive (not deductive) arguments are technically invalid
  • The terms most often used to distinguish good and bad inductive arguments are strong and weak.
An example of a strong inductive argument would be:
    • (1) Every day to date the law of gravity has held.

    • (2) The law of gravity will hold tomorrow.
  • Arguments that fail to meet the standards required of inductive arguments commit fallacies.
  • It is these informal fallacies that we are concerend with
Arguments consist of premises, inferences, and conclusions.
  • Therefore…you will be examining
    • Premises
    • Inferences
    • conclusions
fallacy of relevance
—Fallacy of Relevance—
  • The fallacies of relevance clearly fail to provide adequate reason for believing the truth of their conclusions.
  • They are often used in attempts to persuade people by non-logical means, only the unwary, the predisposed, and the gullible are apt to be fooled by their illegitimate appeals.
  • Many of them were identified by medieval and renaissance logicians, whose Latin names for them have passed into common use.
ad hominem
Ad Hominem

Latin for to the man

Directly attacks someone’s appearance, personal habits, or character rather than focusing on the merit of the issue at hand. The implication is that if something is wrong with this person, whatever he/she says must be wrong.

How can you say he’s a good musician when he’s been in and out of rehab for three years?

ad hominem making it personal
Ad Hominem – Making it Personal

Sara is divorced, so whatever relationship advice she gives you can’t be good.

It is the suggestions, not the person who makes them that deserve attention. Sara’s marital status has nothing to do with the quality of her advice. Isn’t it also possible that Sara could be married and give awful advice?

If my husband forgot to wash his dish, I would move out too. You did the right thing, Carol.

  • The arguer suggests that her opponent’s view is unacceptable because of some negative character trait. Attack the person rather than the argument.
    • People who say that hazing in the military is wrong are just a bunch of wimps.
    • He’s a liar so there’s no reason to listen to him.
    • …But Ginsberg’s arguments are nothing but trash. Ginsberg was a marijuana-smoking homosexual and a thoroughgoing advocate of the drug culture….
    • Humphrey Ad
still more
Still more…
  • Ad hominem : circumstantial
  • The arguer suggests that her opponent’s view is false because the opponent has something personal to gain if it is accepted.
    • Of course France opposed the war on Iraq; they’ve got millions of dollars of contracts at stake.
    • We should disregard that scientist’s argument because they are being funded by the logging industry.
o yeah
O yeah!
  • Ad hominem :: tu quoque (‘You too’)
  • The arguer suggests that her opponent’s position is inconsistent with their own beliefs or actions and therefore the position is false.
    • You’re telling me to stop speeding on the highway? You’ve received more speeding tickets than I have.
    • Gore is a hypocrite on Campaign finance issues – he’s raised as much money as anyone.
    • You say I shouldn't drink, but you haven't been sober for more than a year.
red herring
Red Herring
  • Red Herring: When the arguer changes the subject and take the listener down a different, unrelated path.
    • Environmentalists are continually harping about the dangers of nuclear power. Unfortunately, electricity is dangerous no matter where it comes from. Every year hundreds of people are electrocuted by accident. Since most of the accidents are caused by carelessness, they could be avoided if people would just exercise greater caution.
guilt by association
Guilt By Association
  • The arguer suggests her opponent’s views should be rejected because the opponent is a member of a perceived disreputable group or the views of the opponent are also held by persons of a disreputable group.
    • Nationalized health care programs are unacceptable because they are the sort of thing that Communists support.
    • There’s no reason to deal with Arafat. He’s a known terrorist.
    • Politician X once spoke with the leader of the KKK.
ad populum bandwagon
Ad Populum (Bandwagon)
  • The arguer appeals to the sheer number of persons who agree with the belief or to the popularity of the belief as evidence that it is true.
    • Because a majority of Americans believe in UFO’s, they must exist! That many people can’t be wrong!
    • 4 out of 5 dentists recommend brushing with pure cane sugar.
appeals to pity ad misericordium
Appeals to pity (Ad Misericordium)
  • The arguer tries to get you to accept their view on the grounds that they will be harmed if you don’t.
    • I really need you to give me an A in this course. I know I didn’t do that well, but an average grade will bring my grade point average down.
    • Just do as I ask before you give me a heart attack!
appeals to force fear
Appeals to force/fear
  • The arguer tries to get you to accept their view on the grounds that you will be harmed if you don’t. They use attempt to motivate you from fear rather than logically persuade you.
    • So you’re an animal rights activist. I’d consider changing my views if I were you because most of us here on the prairies are beef farmers and we don’t care too much for your kind.
    • Willie Horton Ad
appeal to tradition
Appeal to Tradition
  • The arguer bases the acceptance of a position on the mere fact that they have always believed it or that it has always been accepted, that it is true.
    • Although horrendous in our eyes, the burning of the wives of deceased men should be considered morally acceptable since the society in question has been doing it for centuries.
fallacies of inference or inductive fallacies
—Fallacies of Inference or Inductive Fallacies—

This category is for inductive fallacies, or faulty generalizations, arguments that improperly move from specific instances to general rules.

post hoc
Post Hoc…
  • Short for post hoc, ergo propter hoc,

which means after this, therefore caused

by this.

This fallacy assumes that just because B

happened after A, it must have been caused

by A.

Politicians love this one.

post hoc example
Post Hoc Example

Since Governor Bush took office, unemployment of minorities in the state has decreased by seven percent. Governor Bush should be applauded for reducing unemployment among minorities.

Before we pat the governor on the back, the speaker must show that Bush’s policies are responsible for the decrease in unemployment. It is not enough to show the decrease came after his election.

ad ignoratium
Ad Ignoratium
  • The arguer uses the fact that a proposition has not been disproved as evidence that the proposition is true, or if it has not been proven, that it is false.
    • People have been trying for centuries to provide conclusive evidence that astrology doesn’t work. But they haven’t. Therefore, we must conclude that the claims of astrology are true.
    • You haven’t disproved that Mossad wasn’t involved in 9/11, which suggests they almost surely were.
    • Since you cannot prove that ghosts do not exist, they probably exist.
    • Since scientists cannot prove that global warming will occur, it probably won't.
hasty generalization
Hasty Generalization

A hasty generalization is a conclusion based on insufficient or unrepresentative evidence.

Stereotyping and Sexism are forms of this fallacy. Take, for example common dumb blonde jokes:

Q: What do you call a blonde skeleton

in the closet?

A: Last year's hide-and-go-seek winner.

example of a generalization
Example of a Generalization

The only redheads I know are rude.

Therefore, all redheads must have bad


If the speaker only knows two redheads, then he has insufficient evidence to make the general claim about all people with that hair color.

more on generalizations
More on Generalizations
  • Any argument that draws a generalization based on a small or unrepresentative sample size.
    • 75% of the people who responded to our Poll via e-mail have some college education, so 75% of all Americans have some college education.
    • You can't speak French; I can't => no one at the UofA can
false analogy
False Analogy

An analogy points out similarities in things that are otherwise different. A false analogy claims comparison when differences outweigh similarities. Essentially, it’s comparing apples and oranges!

false analogy example
False Analogy Example

If we can put a man on the moon, why

can’t we find the cure for the common


While both things being compared here are related to science, there are more differences than similarities between space and biological advancements.

fallacies of false premise or ambiguity
— Fallacies of False Premise or Ambiguity—

Fallacies of ambiguity appear to support their conclusions only due to their imprecise use of language. Once terms are clarified, fallacies of ambiguity are exposed. It is to avoid fallacies of this type that philosophers often carefully define their terms before launching into an argument.

straw man
Straw Man
  • The arguer makes her own position appear stronger by misrepresenting her opponent’s position.
    • Mr. Goldberg has argued against prayer in public schools. Obviously Mr. Goldberg advocates atheism. But atheism is what they used to have in Russia. Atheism leads to the suppression of all religions and the replacement of God by an omnipotent state. Is that what we want for this country? I hardly think so. Clearly Mr. Goldberg’s argument is nonsense.
    • Is this really what Goldberg was arguing. Almost certainly not.
hypothesis contrary to the fact
Hypothesis Contrary to the Fact
  • start with a hypothesis that's not true and draw conclusions from it
    • If Columbus had never found America, the Islanders would not have been decimated by disease.
      • But, someone else would have later!?
either or false dilemma
Either – Or / False Dilemma

A false dilemma asserts that a complex situation can have only two possible outcomes and that one of the options is necessary or preferable.

Either go to college or forget about making money.

This falsely implies that a college education is a pre-requisite for financial success.

Was it her college education that made Britney tons of money?

more false dilemma
More False Dilemma

The arguer claims that there are only two options and one is unacceptable so we must accept the other. However in actuality there are other alternatives.

  • Either we ban negative ads or we let them run amok on our television stations.
  • “Either the Saudis control the US government, or they don’t” – Christopher Hitchens
  • If Guns are outlawed, only outlaws will have guns.
  • Daisy Girl Ad
semantic fallacies
—Semantic Fallacies—
  • Fallacies in this category rely on vague or imprecise language.
slippery slope
Slippery Slope
  • If A happens, then by a gradual series of small steps through B, C,…, X, Y, eventually Z will happen, too. 
Z should not happen. 
Therefore, A should not happen, either.
  • Marajuana is a gateway drug—all heroin addicts started by smoking pot.
    • (yeah, they also all started by drinking milk, so…)
  • [I]f once a man indulges himself in murder, very soon he comes to think little of robbing; and from robbing he comes next to drinking and Sabbath-breaking, and from that to incivility and procrastination. Once begin upon this downward path, you never know where you are to stop. Many a man has dated his ruin from some murder or other that perhaps he thought little of at the time.
affirming the consequent
Affirming the Consequent
  • Any argument with the invalid structure of: If A then B. B, therefore A.
    • If I get a B on the test, then I will have passed. I passed the test, so it follows that I must have received a B.
denying the antecedent
Denying the Antecedent
  • Any argument with the invalid structure of: If A then B. Not A, therefore not B.
    • If it’s a dog then it’s a mammal. It’s not a dog, so it must not be a mammal.
  • The arguer shifts the meaning of a term or phrase from one premise to the next.
    • Insane people are considered not responsible for their actions. Your poor driving habits show a serious lack of responsibility, so you must be insane.
    • Every society is, of course, repressive to some extent - as Sigmund Freud pointed out, repression is the price we pay for civilization.
    • That kid’s a little terror! I hope he doesn’t fly planes into buildings some day!
poisoning the well
Poisoning the Well
  • The arguer uses biased language (either positive or negative) to support their views rather than offering evidence.
    • We should offer our support to Haiti because they are a backward nation and thus require guidance from a progressive country like ours.
examples of such language
Examples of such language
  • Revenue Enhancement' = taxes
  • `protective reaction strike' or `air support' = bombing
  • `pre-dawn tactical insertion' = early morning invasion
  • `incontinent ordinance' = Off-target bombs
  • `friendly fire' = Shelling friendly village or troops mistakenly
  • `Human Remains Pouches' = Body Bags (flexible coffins)
  • Strategic Withdrawal' = retreat
  • `selective ordinance' or `selected chemical insertion' = napalm
  • `Involuntary conversion'(of an aircraft) = plane crash (Eastern Airlines)
  • `public safety unit' = in some countries, a murder squad
  • Collateral damage = dead innocent civilians
begging the question
Begging the Question

This is a kind of circular argument where the support only restates the claim.

Wrestling is dangerous because it is unsafe.

Jogging is fun because it is enjoyable.

Unsafe means the same thing as dangerous and fun means the same thing as enjoyable. This makes the reasoning circular.

put another way
…put another way
  • The arguer supports the conclusion simply by restating it as a premise or by leaving out a key premise.
    • I know she loves me because she told me so, and you don’t lie to someone you love.
    • God exists because the Bible says that he does. We all know that the Bible is accurate because it was written by inspired men, men inspired by God to write down his words.
    • Whatever is less dense than water will float, because such objects won't sink in water.
    • Murder is morally wrong. This being the case, it follows that abortion is morally wrong.
definition too broad
Definition too broad
  • The definition includes items that should not be included
    • An apple is something that is red and round. The planet Mars is red and round. So it is included in the definition. But obviously it is not an apple.
    • A figure is square if and only if it has four sides of equal length. (Not only squares have four sides of equal length; trapezoids do as well.)
definition too narrow
Definition too Narrow
  • The definition does not include items which should be included.
    • An apple is something that is red and round. (Golden Delicious are apples, however, they are not red. They are not included in the definition, however, they should be.)
    • Something is music if and only if it is played on a piano. (A drum solo cannot be played on a piano, yet it is still considered music.)